Modern tanks are generally equipped with a cannon provided with a periscopic sight and a machine-gun, which may also be provided with a periscopic sight. It is generally accepted as a principle of tank design that appurtenances, such as sights, should protrude as little as possible above the tank in order to reduce the risk of detection by an enemy.
Conventional periscopic sights generally include first and second opposing, parallel, reflective surfaces arranged along an optical axis passing through an elongate housing. The housing defines a window associated with the first reflective surface and an eyepiece associated with the second reflective surface. A beam of light entering the periscope through the window strikes the first surface and is deflected along the optical axis onto the second surface and out through the eyepiece. In the instance of a periscopic sight used in conjunction with tank machine-gun, the periscope housing is usually mounted for axial rotation at right-angles to a plane defined by the tank turret and is mechanically `enslaved` to the machine-gun.
Therefore, when the machine-gun is partially rotated in a plane of movement parallel to the plane defined by the tank turret, the periscope is rotated accordingly so as to provide a line of sight parallel to the direction in which the machine-gun is aimed.
The first and second reflective surfaces may be defined either by prisms or by mirrors. Where the first reflective surface is defined by a mirror, there may also be provided apparatus for tilting it relative to the line of sight, this apparatus also including electronic apparatus for coordinating the tilting of the mirror with a change in the degree of tilt of the machine-gun relative to the plane of movement.
It has been found that rotation of the first surface through an angle `a` while maintaining the second surface in a fixed orientation is effective to change the line of sight by 2a. It will, however, be appreciated that as the first surface is tilted, the optical axis is also tilted and that its respective points of intersection with the two optical surfaces move. Therefore, geometrical constraints, related to the size of the reflective surfaces and to the extent to which it is desirable to minimize the height of protrusion of the sight above the tank limit the extent to which a system employing a tiltable first mirror can move the line of sight.
In the field of periscopic sights for tank cannons. wherein the sight is `enslaved` to the cannon in a way similar to the enslavement of a machine-gun sight to a machine-gun, the periscopic sights respectively enslaved to the cannon and to the machine-gun are generally connected to their respective weapons in a way that does not facilitate quick and convenient disconnection of the machine-gun sight from the machine-gun, disconnection of the cannon sight from the cannon and connection of the machine-gun sight to the cannon. This would be particularly useful upon failure of the cannon sight under battle conditions.